Open Firmware – PowerPC Mac’s
Open Firmware is the generic name of firmware complying with IEEE Standard 1275-1994. Similar to a PC’s BIOS, Open Firmware is stored in ROM and is the first stored program to be executed upon power-up.
Open Firmware is available on PowerPC based Macintosh’s. Generally you won’t interact with Open Firmware, however it can be useful in certain situations.
You can enter Open Firmware Mode by pressing Option-Command-O-F at boot.
Below are Open Firmware commands that can come in handy.
mac-boot
This command boots Mac OS on an Open Firmware 3 system (Any PowerPC Mac)
reset-nvram
This command does exactly what it says, resets the nonvolatile RAM. This should be followed by reset-all. If you cannot reset the PRAM/NVRAM by holding down Option-Command-P-R at boot, then try this.
reset-all
This command permanently stores the Open Firmware environment variables and reboots the computer.
printenv
This prints (to screen) the current and default values of the settings in the Open Firmware environment.
Extended Firmware Interface – Intel Mac’s
EFI is a replacement for PC BIOS that was developed mainly because BIOS was limited to only x86 CPUs in an era when Intel was branching off to other platforms such as Itanium. Of note, Apple has implemented popular Open Firmware commands, such as Target Disk Mode (T key held down during startup) and startup disk selection (option key held down during startup) in EFI on Intel Macs.
The EFI command interface is not available for access unless you install a third-party program such as rEFIt.
If the EFI is messed with, loading modules, etc, an Intel Mac can become unbootable. To recover an Intel-based Mac from an unbootable state, the following steps may be useful:
- Unplug all cables from the computer, including the power cord.
- Wait 10 seconds.
- Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer (this resets the System Management Controller (SMC), similar to resetting the System Management Unit (SMU) on a G5 class machine).
- Let go of the power button.
- Reconnect the mouse and keyboard.
- Press the power button to start up your Mac, and reset the NVRAM by holding command-option-P-R at boot time, waiting until a second startup chime is heard.
- If the Mac still does not boot from its hard disk, you may need to boot from the DVD installation media, reformat the drive, and restore the operating system.
If the above steps do not work, you can try to restore the default firmware to the machine. Download and burn Apple’s Firmware Restoration CD and run it. It will reset the default firmware on the Intel based Mac. More info and a download link for the CD is available here.
Useful Links
Pingback: Linux installation messed up Mac OS